1. Field of the Invention
The present invention relates to an etching method of a silicon wafer surface and an apparatus of etching thereof in order to analyze an exceedingly slight quantity of metal impurities contained in the silicon wafer surface, in a thin film formed on the silicon wafer surface, or a surface layer of the silicon wafer.
More particularly, this invention relates to an etching method and an apparatus of etching thereof in order to analyze an extremely slight quantity of metal impurities contained in the surface layer of the silicon wafer.
2. Description of the Prior Art
It has been known that metal impurities in a silicon water surface, a surface layer of a silicon wafer, or a thin film formed on the surface affect adversely on electrical characteristics of semiconductor devices. For this reason, Analyzing methods of metal impurities in the silicon wafer surface, the surface layer, or the thin film such as a thermal oxide film have been heretofore investigated.
A representative method which has been investigated is called a vapor phase decomposition method. In Japanese Patent Publication No. 58927/1994, an analyzing method of impurities in a semiconductor thin film and a recovery apparatus of a decomposed liquid for an analysis is disclosed wherein a semiconductor wafer is arranged vertically within an airtight chamber, a thin film formed on the surface of the semiconductor wafer is decomposed with hydrogen fluoride vapor, and decomposed liquid is collected in a collection container disposed in the lower portion of the airtight chamber to be analyzed.
Furthermore, Japanese Patent Publication No. 43289/1993 discloses a method of analyzing a semiconductor thin film and an apparatus thereof wherein a semiconductor wafer is arranged horizontally, only one side surface of the semiconductor wafer is exposed to hydrogen fluoride vapor, and a thin film is decomposed.
All of these inventions use hydrogen fluoride gas for decomposing the thin film formed on the surface of the semiconductor wafer. Hydrogen fluoride gas is able to decompose the thin film such as a natural oxide film and a thermal oxide film formed on the silicon wafer. However, hydrogen fluoride gas is impossible to etch silicon itself so that impurities contained in the vicinity of the surface of the wafer, that is, impurities in surface layer can not be recoverd.
For an evaluation method of metal impurities in a silicon wafer surface layer, Japanese Patent Publication No. 44830/1993 and No. 31940/1993 disclose a method wherein a low temperature oxidation is performed in a specific reaction apparatus and a vapor phase decomposition of the formed oxide film is subsequently carried out. There is a drawback that this method requires the specific reaction apparatus, and that because respective elements consisting in metal impurities are introduced into the oxide film in different ways, some of the elements can not be evaluated with a high sensitivity in this method.
On the other hand, a method has been known wherein a silicon wafer is etched with mixed acid made of hydrofluoric acid and nitric acid, etching solution is collected, and subsequently metal impurities contained in the collected liquid are analyzed. In Japanese Patent Publication No. 30209/1993 and Japanese Patent Publication No. 57422/1991, for example, a sample decomposition apparatus and a sample decomposition method using this apparatus are disclosed wherein a container containing pure water in which a silicon wafer is dipped is located in an airtight chamber, acidic gas is produced by heating a hydrofluoric acid and nitric acid container in the airtight chamber, and metal impurities dissolved into the pure water by etching the wafer are analyzed with a frame-less atomic absorption spectrometry apparatus.
However, as clearly remarked from that an etching is carried out as to the whole portion of a silicon wafer in the embodiment, a silicon wafer is dipped in pure water when performing this method. Therefore, controlling of the etching amount is difficult so that it is impossible to analyze the wafer surface layer with a precision.
To solve this problem, Japanese Patent Application Laid Open 211223/1993 discloses a method to analyze only the silicon water surface layer. Specifically, a silicon wafer is horizontally located in an airtight chamber, and a silicon piece as a catalyzer for producing gas is dipped in mixed acid composed of hydrofluoric acid and nitric acid thereby producing mixed gas composed of Nox and HF. An etching of the silicon wafer surface is carried out by the mixed gas, and pure water is dropped on the surface after being subjected to the etching. Metal impurities are collected by recovering the water, and the impurities are subjected to atomic absorption spectrometry.
Furthermore, Japanese Patent Application Laid Open 213805/1994 discloses a method wherein after the contents of nitric acid and hydrogen fluoride contained in mixed acid are normalized, the surface of a silicon wafer is etched by heating the mixed acid, condensed liquid is collected without using chemicals, and the liquid is analyzed.
With the foregoing methods, a large amount of hydrogen fluoride gas is produced at the initial stage of the reaction (the reason for the action is unknown), and the surface of the silicon wafer is terminated with hydrogen by hydrogen fluoride so that the surface of the silicon wafer is partially rendered hydrophobic. Uneven etching is apt to be produced so that the surface of the silicon wafer is not etched uniformly. Or else, surface roughness is produced. These defects are considered to be caused by the above behavior. When the surface roughness is produced on the surface of the silicon wafer after etching, scanning of a pure water drop on the surface of the silicon wafer is difficult at the time of the collection of metal impurities using a pure water drop disclosed in Japanese Patent Application Laid Open 211223/1993. Furthermore, collection of dew condensed liquid by blowing cleaning gas onto the surface of the silicon wafer, disclosed in Japanese Patent Application Laid Open 213805/1994, is difficult.